Silicon (Si) photonics technology is about to enable optical interconnection devices and transmit/receive circuits to be integrated at high densities and mass-produced at low costs. However, light sources made of an indirect transition material, such as Si, SiGe or Ge, exhibit extremely low emission efficiency. Therefore, semiconductor lasers that have a light-emitting layer made of a III-V semiconductor material are commonly used for light sources. When a III-V semiconductor laser is mounted on a Si substrate or an SOI (silicon on insulator) substrate, a flip-chip mounting technique can be used. With this method, however, light emitted from the semiconductor laser cannot be easily coupled to a fine Si waveguide with low loss and a high yield. In order to overcome this disadvantage, Si hybrid lasers have been proposed. These lasers employ an integrated waveguide structure in which a III-V active layer is bonded to a Si substrate directly or with an ultrathin insulating layer therebetween.
When optical signals are transmitted stably at as fast as several to several tens of Gbps, single-mode lasers, such as distributed feedback (DFB) or distributed Bragg reflector (DBR) lasers, are desirably used. It can be said that lasers of these types are suitably integrated on a Si substrate, because they do not require reflecting end surfaces that cannot be formed easily on the Si substrate, and enables emitted light to be coupled stably to a Si waveguide at high efficiency with a taper structure. When lasers having a wide tunable wavelength range are required, DBR lasers, including SSG (super-structured grating) DBR lasers and SG (sampled-grating) DBR lasers, in which the refractive index of the diffraction grating provided in a passive waveguide is tunable, are desirably used. However, if being applied to on-chip optical interconnections that do not need a wide tunable wavelength range, DFB lasers are more desirable in terms of their compactness and efficiency.
In order to avoid an instability of longitudinal modes that would be caused due to an occurrence of axial hole burning upon a high injection, it is desirable that a λ/4 phase shifter be provided in a DFB laser at its center and the product κL of the coupling coefficient κ of the diffraction grating and the resonator length L be set to 1.25 or so. A DFB laser needs to oscillate in a fundamental (zero-order) transverse mode, so that its emitted light can be coupled to a Si waveguide efficiently. In addition, it is necessary to sufficiently attenuate return light reflected at the output end of the waveguide such that mode instability can be avoided.
The majority of Si hybrid DFB lasers reported are of Si guiding type (evanescent lasers) in which light that propagates along a waveguide formed on a Si layer is evanescently coupled to a III-V active layer. In a typical Si guiding type of laser, a thick Si rib waveguide (more than 500 nm thick) is used to confine a major part of the fundamental mode in the Si waveguide, and the current path in a wide III-V mesa is constricted so as to align with the Si rib waveguide by means of proton ion implantation, for example. Si guiding types of lasers configured above have advantages in that: emitted light can easily be coupled to an output Si rib waveguide; the contact resistance to an electrode on a wide III-V mesa is low; and heat can be dissipated via the wide III-V mesa. However, they have disadvantages in that: the confinement factor Γ of the active layer is small, which results in low modal gain and slope efficiency; the coupling coefficient κ of the diffraction grating tends to be excessively large (>100 cm−1, typically), which results in a strong axial hole burning; and a thick Si rib waveguide cannot be bent with a small radius, which makes it difficult to increase the density of optical interconnections.
A supermode type of Si hybrid laser has been proposed as a structure that improves the above disadvantages. In this type of laser, light is primarily confined in a III-V semiconductor waveguide in the laser section, and it is gradually transferred into an output Si waveguide in an adjoining coupling section. In a DFB laser, however, a Si waveguide with a diffraction grating thereon is required also in the laser section. Because of a complex waveguide structure extending over the III-V semiconductor waveguide and the Si waveguide, multiple modes exist therein. When difference in effective indexes of the modes in the two original waveguides is small, these two modes are combined and transformed into a pair of supermodes. In order to achieve high slope efficiency, it is required that the laser oscillates in the fundamental supermode with a relatively large Γ and κ. In this case, unfortunately, in addition to higher-order modes in the active layer having a large Γ, higher-order modes in the Si waveguide having a large κ and the other supermode having a relatively large Γ and κ compete with the fundamental mode. Furthermore, there is difficulty controlling the characteristics of the supermode, because the confinement factor Γ and coupling coefficient κ of the supermode are highly sensitive to structural parameters.
A III-V guiding type of Si hybrid laser has been proposed as a structure that can improve the above disadvantage. In this type of laser, a Si waveguide in the laser section is replaced by a Si slab layer. Since no guided mode exists in the Si slab layer, modes can be controlled much easier than supermode types. By forming a diffraction grating on a surface of the Si slab layer under the center of a III-V mesa (at a location of a node of the first-order mode) such that it is sufficiently narrower than the III-V mesa, the coupling coefficient of the diffraction grating for the first-order mode (abbreviated below as a “first-order mode coupling coefficient”) can be made sufficiently smaller than that for the fundamental (zero-order) mode. If the III-V mesa width is decreased such that second and higher order modes do not appear, single mode oscillation in the fundamental transverse mode is easily realized. Since the confinement factor Γ in this type of laser is higher than that in a Si guiding type as described above, the slope efficiency is higher. Moreover, the coupling coefficient of the diffraction grating κ can be controlled easily within a proper range. Therefore, a stable single mode operation without axial hole burning is realized over a wide range of operating condition. By utilizing a thin Si slab layer, light can be emitted to a Si waveguide with a small cross section (e.g., with 220 nm thick and 450 nm wide) through an output taper. This is advantageous to downsize optical integrated circuits and increase their package densities.
In a Si hybrid laser, a thick buried SiO2 (BOX (buried oxide)) layer (with more than 1 μm thick) is formed under a top Si layer. This layer disadvantageously increases the thermal resistance and degrades the thermal characteristics. It is reported that a Si guiding type of Si hybrid laser that employs a thermal shunt structure in which polysilicon is embedded in the BOX layer under a semi-insulating part of the InP mesa can decrease its thermal resistance to 40 K/W or less. In a III-V guiding type of Si hybrid laser, however, the III-V mesa cannot be widened and accordingly its thermal resistance cannot be decreased easily. It is thus necessary to improve the thermal characteristics of III-V guiding types of Si hybrid lasers.